Heart Rate Zones (Karvonen) Calculator

Heart Rate Training Zones

Heart rate zones split aerobic effort into ranges of percentage of maximum heart rate (HRmax) or heart rate reserve (HRR). HRmax can be estimated by the classical Fox equation HRmax = 220 - age or by the more accurate Tanaka 2001 formula HRmax = 208 - 0.7 × age. The Karvonen method refines this by subtracting resting heart rate: target = (HRmax - HRrest) × %intensity + HRrest.

The five-zone model is the standard: Z1 50-60% active recovery, Z2 60-70% fat-burning and aerobic base, Z3 70-80% aerobic capacity, Z4 80-90% lactate threshold / anaerobic, Z5 90-100% VO₂max and neuromuscular work. Each zone elicits a distinct physiological adaptation.

Applications

Endurance coaches frequently use the 80/20 polarised model: roughly 80% of training volume in Z1-Z2 and 20% in Z4-Z5, with little time in the “grey” Z3. This approach, popularised by Stephen Seiler, maximises aerobic base while keeping cumulative fatigue low and is widely adopted in running, cycling and triathlon programming.

FAQ

Fox or Tanaka? Tanaka (208 - 0.7×age) is more accurate, especially over 40. Fox (220 - age) is simpler but tends to underestimate HRmax in older adults.

Do I need a chest strap? Chest straps are more accurate than wrist optical sensors, especially during interval work. For Z2 and recovery, wrist sensors are usually good enough.

Is Z2 really “fat burning”? Yes — at 60-70% HRmax fats supply most of the energy, but total calorie burn is lower than at higher intensities. Z2 builds the mitochondrial base.